Method for evaluating carbon concentration

ABSTRACT

A method for evaluating a carbon concentration where ions of a predetermined element are implanted into a silicon wafer, and then a carbon concentration is measured by a low-temperature PL method from an emission intensity of a CiCs composite, where the ions are implanted under implantation conditions of 1.1×10 11 ×[atomic weight of the implanted element] −0.73 &lt;implantation amount (cm −2 )&lt;4.3×10 11 ×[atomic weight of the implanted element] −0.73 , and the carbon concentration is evaluated. A method for evaluating a carbon concentration makes it possible to measure with high sensitivity, a carbon concentration in a surface layer of 1 to 2 μm, which is a photodiode region in an image sensor.

TECHNICAL FIELD

The present invention relates to a method for evaluating a carbonconcentration.

BACKGROUND ART

As a substrate for fabrication of a semiconductor integrated circuit, asilicon wafer fabricated by a CZ (Czochralski) method is mainly used.Failures in the most advanced image sensors in recent years areconsidered to be caused by a very small amount of metal impurity in adevice active region. Specifically, a metal impurity in a wafer causes awhite flaw failure by forming a deep level. Meanwhile, there is an ionimplantation process in a device process, and point defects areproduced. The point defects that are produced by ion implantation reactwith carbon in the wafer to form CiCs composites. A CiCs composite formsa level, and therefore, is predicted to be a cause of white flawfailure.

Ways of measuring a carbon concentration in a wafer include an FT-IRmethod, SIMS, and electron beam irradiation+low-temperature PLmeasurement.

CITATION LIST Patent Literature

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2015-222801

Non Patent Literature

Non Patent Document 1: JEITA EM-3503

Non Patent Document 2: Applied Physics, volume 84, No. (2015) p.976

SUMMARY OF INVENTION Technical Problem

The above-mentioned FT-IR method is a method of transmitting infraredrays into a wafer and quantifying a carbon concentration from aninfrared absorption peak in a 1106 cm⁻¹ position (Non Patent Document1). However, analysis by the FT-IR method is an evaluation of the waferin the entire depth direction since measurement is performed byabsorption of transmitted light, and therefore, an outermost surfacelayer, which is an active layer of a device, cannot be evaluated.Moreover, sensitivity is low and the lower detection limit isapproximately 0.03 ppma, which is high.

In the above-mentioned secondary ion mass spectrometry (SIMS), anelemental analysis can be performed by irradiating a sample surface withprimary ions and performing a mass spectrometry on emitted secondaryions. Distribution of a carbon concentration in the depth direction canbe measured by analysis using SIMS, but the lower detection limit isapproximately 0.05 ppma, and analysis of very low carbon is difficult.

The above-mentioned electron beam irradiation+low-temperature PLmeasurement method is a method in which point defects introduced into awafer by electron beam irradiation react with carbon to form compositesof interstitial carbon and substituted carbon (CiCs composites), and acarbon concentration is quantified by the low-temperature PL method froman intensity of an emission caused by the composites (Non PatentDocument 2).

This technique has favorable sensitivity and a low lower detectionlimit, but CiCs composites are formed in the entire depth direction ofthe wafer, and CiCs composites that are detected by the low-temperaturePL method depend on the diffusion depth of carriers duringlow-temperature PL measurement. For example, in Non Patent Document 2,the depth is about 10 μm, and a carbon concentration in a shallowerregion cannot be measured. For example, a surface layer of 1 to 2 μm,which is a photodiode region of an image sensor cannot be evaluated.

In addition, preceding technology for measuring a carbon concentrationby ion implantation+PL method includes Patent Document 1, and this is amethod for creating a calibration curve for quantification. Furthermore,ion implantation conditions for forming CiCs composites to detect by alow-temperature PL method in measuring the carbon concentration are notoptimized in view of measuring the carbon concentration with highsensitivity.

The present invention has been made in view of the above-describedproblems, and an object thereof is to provide a method for evaluating acarbon concentration that makes it possible to measure with highsensitivity, a carbon concentration in a surface layer of 1 to 2 μm,which is a photodiode region in an image sensor.

Solution to Problem

To achieve the object, the present invention provides a method forevaluating a carbon concentration where ions of a predetermined elementare implanted into a silicon wafer, and then a carbon concentration ismeasured by a low-temperature PL method from an emission intensity of aCiCs composite, wherein the ions are implanted under implantationconditions of 1.1×10¹¹×[atomic weight of the implantedelement]^(−0.73)<implantation amount (cm⁻²)<4.3×10¹¹×[atomic weight ofthe implanted element]^(−0.73), and the carbon concentration isevaluated.

In this manner, the emission intensity of the CiCs composite in asurface layer can be raised by implanting the ions under theimplantation conditions of 1.1×10¹¹×[atomic weight of the implantedelement]^(−0.73)<implantation amount (cm⁻²)<4.3×10¹¹×[atomic weight ofthe implanted element]^(−0.73). Therefore, it is possible to measurewith high sensitivity, a carbon concentration in a surface layer of 1 to2 μm, which is a photodiode region in an image sensor. Consequently, awafer with favorable image sensor characteristics can be selected. Inaddition, by making it possible to measure the carbon concentration inthe surface layer in stages in the middle of the process as well, acontamination state of the carbon concentration in various processes canbe grasped easily.

In this event, it is preferable that a recovery heat treatment isperformed at a temperature of 200° C. or less after the ions areimplanted, and then a carbon concentration is measured by alow-temperature PL method from an emission intensity of a CiCscomposite.

By thus performing a recovery heat treatment at a temperature of 200° C.or less after the ions are implanted, the emission intensity of the CiCscomposite can be raised more effectively.

In this event, an element to ion-implant into the silicon wafer may behelium or hydrogen.

Such elements can be used suitably as an element to ion-implant into thesilicon wafer.

In this event, the carbon concentration is preferably measured in aregion within a range of 2 μm or less from a surface of the siliconwafer.

By measuring the carbon concentration of such a region, a carbonconcentration in a surface layer of 1 to 2 μm, which is a photodioderegion in an image sensor can be measured with high sensitivity, and awafer with favorable image sensor characteristics can be selected.

Advantageous Effects of Invention

As described above, according to the inventive method for evaluating acarbon concentration, an emission intensity of a CiCs composite can beraised, and therefore, it is possible to measure with high sensitivity,a carbon concentration in a surface layer of 1 to 2 μm, which is aphotodiode region in an image sensor. Consequently, a wafer withfavorable image sensor characteristics can be selected. In addition, bymaking it possible to measure the carbon concentration in the surfacelayer in stages in the middle of the process as well, a contaminationstate of the carbon concentration in various processes can be graspedeasily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram showing the inventive method for evaluating acarbon concentration.

FIG. 2 is a graph showing the difference in emission intensities of CiCscomposites in cases where He ion implantation, He ionimplantation+additional heat treatment (150° C./60 min), and O ionimplantation were used.

FIG. 3 is a figure showing low-temperature PL spectra before and afteretching.

FIG. 4 is a figure showing the emission intensity of CiCs composites ina low-temperature PL measurement after ion implantation of each element.

FIG. 5 is a figure showing dependence of CiCs composite emissionintensity on an implantation amount of He ion implantation.

FIG. 6 is a figure showing implantation conditions under which CiCscomposite emission intensity becomes high.

FIG. 7 is a graph showing a relation between an additional heattreatment time after ion implantation and CiCs composite emissionintensity.

FIG. 8 is a graph showing a relation between an additional heattreatment temperature after ion implantation and CiCs composite emissionintensity.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in detail as anexample of an embodiment with reference to the drawings, but the presentinvention is not limited thereto.

Firstly, it will be explained in the following that after implantingions, a carbon concentration in a wafer surface layer can be measured bya low-temperature PL method.

Point defects are introduced into a wafer by ion implantation. Theintroduced point defects react with carbon in the wafer to form CiCscomposites. The formed CiCs composites can be detected by thelow-temperature PL method, and when a conductivity type is the same, anda resistivity and an oxygen concentration are similar, the carbonconcentration can be quantified.

Other than C, point defects also react with oxygen and dopants in thewafer. Accordingly, attention needs to be paid when measuring a carbonconcentration in a wafer with a different oxygen concentration or dopantconcentration.

In addition, point defects produced by ion implantation can be limitedlocally. Specifically, a depth region in which point defects areproduced can be selected according to an implantation energy of the ionimplantation. The depth is approximately a few μm, and the lower theenergy, the shallower. Accordingly, the carbon concentration in thewafer surface layer can be measured.

Firstly, an investigation was conducted to confirm that the CiCscomposites formed by ion implantation were limited to the surface layer.Ion implantation with an energy such that the range is 0.5 μm (He ionimplantation, implantation amount (hereinafter also referred to asdosage): 1×10¹¹ cm⁻², energy: 150 keV) was performed on an n/n-EPW(epitaxial wafer) having an epitaxial layer thickness of 5 μm, and thenevaluation was carried out by the low-temperature PL method.Subsequently, a surface layer of 1 μm was removed by etching, andevaluation was once again performed by the low-temperature PL method.The measurement results of the low-temperature PL method before andafter the etching are shown in FIG. 3. FIG. 3 (a) shows an entirelow-temperature PL spectrum, and FIG. 3 (b) shows an enlarged emissionregion of the CiCs composites. It was confirmed that emission of CiCscomposites (emission that appears around 1278 nm) was not detected afteretching.

That is, it was revealed that the carbon that reacts with the pointdefects that were introduced by the ion implantation performed on thisoccasion was within a surface layer of 1 μm. From this result, it wasrevealed that the carbon that is evaluated by an ionimplantation+low-temperature PL method is in a surface layer ofapproximately the range in the ion implantation.

In addition, to quantify the carbon concentration in the surface layerto a low concentration, a high emission intensity of the CiCs compositesin the low-temperature PL measurement is advantageous. That is, it isnecessary to determine ion implantation conditions that can raise theemission intensity of the CiCs composites.

Next, how the ion implantation conditions (implantation amount) that canraise the emission intensity of the CiCs composites were determined willbe explained below.

A plurality of silicon wafers were prepared, ions of H, He, B, and Owere implanted into each silicon wafer, and then measurement by thelow-temperature PL method was performed. The results are shown in FIG.4. The implantation amount was 1.0×10¹¹ cm⁻² in every ion implantation.As a result, it was revealed that the emission intensity of the CiCscomposite is highest with an ion implantation of He.

Next, ion implantation of He was investigated with differentimplantation amounts. The specific implantation amounts were 1.0×10¹¹ to1.0×10¹⁴ cm⁻². Measurement was performed by the low-temperature PLmethod. The results are shown in FIG. 5. It was revealed from FIG. 5that the emission intensity of the CiCs composites becomes higher as theimplantation amount becomes lower.

From the above two results, the emission intensity of the CiCs compositewith an ion implantation of He at an implantation amount of 1.0×10¹¹cm⁻² was the highest, and at the same implantation amount, with ionimplantation of H, which has a lighter mass than He and causes lowerimplantation damage, the emission intensity of the CiCs compositesbecomes somewhat small, but a high emission intensity of the CiCscomposites can still be obtained. That is, it has been revealed thatoptimum ion implantation conditions exist in this region.

Thereupon, the amount of the point defects produced by the ionimplantation was estimated using a binary collision simulation softwareSRIM (http://www.srim.org/). As a result, it was estimated that when Heis ion-implanted at an implantation amount of 1.0×10¹¹ cm⁻², the averagevacancy concentration in a surface layer of 1 μm is about 1.8×10¹⁷ cm⁻³,and when H is ion-implanted at an implantation amount of 1.0×10¹¹ cm⁻²,the average vacancy concentration in a surface layer of 1 μm is about1.5×10¹⁶ cm⁻³. That is, it has been revealed that ion implantationconditions where the average vacancy concentration is within the rangeof 1.5×10¹⁶ cm⁻³ to 1.8×10¹⁷ cm⁻³ are conditions for raising theemission intensity of the CiCs composites. It has been revealed thatwhen the ion implantation conditions that satisfy these conditions areexpressed by the atomic weight of the implanted element and theimplantation amount,1.1×10¹¹×[atomic weight of the implanted element]^(−0.73)<implantationamount (cm⁻²)<4.3×10¹¹×[atomic weight of the implanted element]^(−0.73)holds. When the ions are implanted under the above-described conditions,the emission intensity of the CiCs composites becomes high. This can beshown in a graph as in FIG. 6. In FIG. 6, the shaded region is theregion in which the emission intensity of the CiCs composites becomeshigh (effective region).

A reason for there being optimum ion implantation conditions asdescribed can be considered as follows. To form CiCs composites,introduction of point defects is necessary, but when the amount of theintroduced point defects is excessive compared with the carbonconcentration, the CiCs composites further react with the point defects,and the emission intensity of the CiCs composites becomes low.Accordingly, it can be presumed that an optimum introduction amount ofpoint defects exists.

Furthermore, it was investigated whether the CiCs emission intensity canbe raised by an additional heat treatment. Specifically, after He ionimplantation at an implantation amount of 1.0×10¹¹ cm⁻², a heattreatment was performed at 50 to 300° C. for 10 to 300 min, and then,the emission intensity of the CiCs composites was investigated by thelow-temperature PL method. The results are shown in FIGS. 7 and 8. FromFIGS. 7 and 8, it was revealed that by adding a heat treatment of 150°C./60 min, the emission intensity of the CiCs composite becomes highest.A reason for this can be considered to be that formation of CiCscomposites was promoted by the additional heat treatment after the ionimplantation. It can be considered that at 250° C. or more, some of theCiCs composites come to be annihilated.

Next, the inventive method for evaluating a carbon concentration will bedescribed in the following with reference to FIG. 1.

FIG. 1 is a flow diagram showing the inventive method for evaluating acarbon concentration.

Firstly, a silicon wafer to be evaluated is prepared (see S11 of FIG.1).

Next, ions of a predetermined element are implanted into the preparedsilicon wafer under implantation conditions of 1.1×10¹¹×[atomic weightof the implanted element]^(−0.73)<implantation amount(cm⁻²)<4.3×10¹¹×[atomic weight of the implanted element]^(−0.73) (seeS12 of FIG. 1).

The element to ion-implant into the silicon wafer is not particularlylimited, but for example, helium or hydrogen is possible.

Such elements can be used suitably as an element to ion-implant into thesilicon wafer.

Next, regarding the ion-implanted silicon wafer, the carbonconcentration is measured by the low-temperature PL method from theemission intensity of the CiCs composites (see S13 of FIG. 1).

In the carbon concentration measurement in S13 of FIG. 1, a region in arange of 2 μm or less from the surface of the silicon wafer ispreferably measured.

By measuring the carbon concentration of such a region, a carbonconcentration in a surface layer of 1 to 2 μm, which is a photodioderegion in an image sensor can be measured with high sensitivity, and awafer with favorable image sensor characteristics can be selected. Notethat the region to measure the carbon concentration can be any region aslong as it is in the range of 2 μm or less from the surface of thesilicon wafer, and the lower limit thereof can be 0 μm (the surface ofthe silicon wafer).

After implanting the ions in S12 of FIG. 1, it is preferable to performa recovery heat treatment at a temperature of 200° C. or less, and thenperform the carbon concentration measurement of S13 of FIG. 1.

By thus performing a recovery heat treatment at a temperature of 200° C.or less after the ions are implanted, the emission intensity of the CiCscomposite can be raised more effectively. Note that the recovery heattreatment can be performed at a temperature of 50° C. or more and 200°C. or less, for example.

According to the inventive method for evaluating a carbon concentrationdescribed above, the emission intensity of the CiCs composites in thesurface layer can be raised, and therefore, it is possible to measurewith high sensitivity, the carbon concentration in the surface layer of1 to 2 μm, which is a photodiode region in an image sensor.Consequently, a wafer with favorable image sensor characteristics can beselected. In addition, by making it possible to measure the carbonconcentration in the surface layer in stages in the middle of theprocess as well, a contamination state of the carbon concentration invarious processes can be grasped easily.

EXAMPLE

Hereinafter, the present invention will be described more specificallywith reference to Examples and Comparative Examples, but the presentinvention is not limited thereto.

Example 1

Ions of He were implanted into n-type silicon epitaxial wafers havingvaried C concentrations in an epitaxial layer at an implantation amountof 1×10¹¹ cm⁻², and then an emission intensity of a CiCs composite wasmeasured by a low-temperature PL method.

Note that the atomic weight of He is 4, and the above implantationamount was within the range of 1.1×10¹¹×[atomic weight of the implantedelement]^(−0.73)=3.99×10¹⁰<implantation amount (cm⁻²)<4.3×10¹¹×[atomicweight of the implanted element]^(−0.73)=1.56×10¹¹.

In addition, in a case where ions were implanted under the sameconditions as those described above and then a further heat treatmentwas performed at 150° C./60 min, the emission intensity of the CiCscomposite was measured by the low-temperature PL method.

Next, the carbon concentration in the epitaxial layer of these siliconepitaxial wafers was measured by SIMS.

A calibration curve of the carbon concentration was then created fromthe relation between the emission intensity of the CiCs composite andthe carbon concentration obtained from these measurement results (seeFIG. 2).

These carbon concentrations show from the ion implantation conditions,the carbon concentrations at approximately 1 μm from the surface layer,and it was revealed that it becomes possible to measure carbonconcentrations of less than 0.002 ppma (see FIG. 2). Furthermore, theemission intensity of the CiCs composites became somewhat higher whenthe heat treatment was performed (see FIG. 2). Thus, it was confirmedthat the sensitivity of carbon concentration detection becomes evenhigher by performing a heat treatment.

Comparative Example 1

Ions of O were implanted into n-type silicon epitaxial wafers havingvaried C concentrations in an epitaxial layer at an implantation amountof 1×10¹¹ cm⁻², and then an emission intensity of a CiCs composite wasmeasured by the low-temperature PL method.

Next, the carbon concentration in the epitaxial layer of these siliconepitaxial wafers was measured by SIMS.

A calibration curve of the carbon concentration was then created fromthe relation between the emission intensity of the CiCs composite andthe carbon concentration obtained from these measurement results (seeFIG. 2).

These carbon concentrations show from the ion implantation conditions,the carbon concentrations at approximately 1 μm from the surface layer.Compared with the cases in Example 1, where ions of He were implanted,the emission intensity of the CiCs composites was small, sensitivity ofcarbon concentration detection was poor, and it was revealed that thecarbon concentration in the surface layer of the epitaxial layer lessthan 0.02 ppma cannot be measured (see FIG. 2).

In this event, to obtain a calibration curve by which carbonconcentrations of less than 0.002 ppma can be measured, the implantationamount needs to be 1.45×10¹⁰(=1.1×10¹¹×[atomic weight of the implantedelement]^(−0.73)) cm⁻²<implantation amount <5.68×10¹⁰ (=4.3×10¹¹×[atomicweight of the implanted element]^(−0.73)) cm⁻², since the atomic weightof O is 15.99.

Example 2 and Comparative Examples 2 and 3

The range of the dosage (implantation amount) of the He ion implantationin the present invention is 4×10¹⁰ cm⁻²<dosage (implantationamount)<1.6×10¹¹ cm⁻², since the atomic weight of He is 4.

Accordingly, in the same manner as in Example 1, He ions were implantedinto n/n⁻EPWs (epitaxial wafers) at a dosage of 3×10¹⁰ (ComparativeExample 2), 5×10¹⁰ (Example 2), and 5×10¹¹ (Comparative Example 3) cm⁻²,and then the emission intensity of a CiCs composite was measured by thelow-temperature PL method.

As a result, assuming that the emission intensity of the CiCs compositewas 1 when the dosage was 1×10¹¹ cm⁻² (Example 1), the emissionintensity was 0.6 with a dosage of 3×10¹⁰ cm⁻², 0.98 with a dosage of5×10¹⁰ cm⁻², and 0.4 with a dosage of 5×10¹¹ cm⁻².

From the above results, it was shown that the emission intensity of CiCscomposites becomes high with a dosage (implantation amount) in the rangeof the present invention.

A reason why the emission intensity of CiCs composites becomes low whenthe dosage is low is that the amount of point defects necessary for thecarbon in a measurement region to form CiCs composites is insufficient.Meanwhile, a reason why the emission intensity of CiCs compositesbecomes low when the dosage is too high is that the amount of pointdefects introduced by implantation becomes excessive and CiCs compositesand point defects react, lowering the concentration of the CiCscomposites.

It should be noted that the present invention is not limited to theabove-described embodiments. The embodiments are just examples, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept disclosedin claims of the present invention are included in the technical scopeof the present invention.

The invention claimed is:
 1. A method for evaluating a carbonconcentration where ions of a predetermined element are implanted into asilicon wafer, and then a carbon concentration is measured by alow-temperature PL method from an emission intensity of a CiCscomposite, wherein the ions are implanted under implantation conditionsof 1.1×10¹¹×[atomic weight of the implantedelement]^(−0.73)<implantation amount (cm⁻²)<4.3×10¹¹×[atomic weight ofthe implanted element]^(−0.73), and the carbon concentration isevaluated.
 2. The method for evaluating a carbon concentration accordingto claim 1, wherein a recovery heat treatment is performed at atemperature of 200° C. or less after the ions are implanted, and then acarbon concentration is measured by a low-temperature PL method from anemission intensity of a CiCs composite.
 3. The method for evaluating acarbon concentration according to claim 2, wherein an element toion-implant into the silicon wafer is helium or hydrogen.
 4. The methodfor evaluating a carbon concentration according to claim 3, wherein thecarbon concentration is measured in a region within a range of 2 μm orless from a surface of the silicon wafer.
 5. The method for evaluating acarbon concentration according to claim 2, wherein the carbonconcentration is measured in a region within a range of 2 μm or lessfrom a surface of the silicon wafer.
 6. The method for evaluating acarbon concentration according to claim 1, wherein an element toion-implant into the silicon wafer is helium or hydrogen.
 7. The methodfor evaluating a carbon concentration according to claim 6, wherein thecarbon concentration is measured in a region within a range of 2 μm orless from a surface of the silicon wafer.
 8. The method for evaluating acarbon concentration according to claim 1, wherein the carbonconcentration is measured in a region within a range of 2 μm or lessfrom a surface of the silicon wafer.